1. Field of the Invention
This invention relates to magnetic recording tapes for audio use, and particularly to those tapes having audio recording and reproducing characteristics with high fidelity.
2. Description of the Prior Art
Typical magnetic audio-recording tape formats include tapes 1/4 inch (6.25 mm) wide and about 18 to 35 microns thick for open reel tapes and tapes about 3.8 mm wide and about 9 to 18 microns thick installed in Philips type compact cassettes. Tapes of these formats can also be installed in EL cassettes, micro cassettes, Lear jet type endless cartridges, etc. Such a tape comprises a non-magnetizable flexible support about 4 to 23 microns thick and a magnetic coating layer about 2 to 12 microns thick. The magnetic coating layer contains ferromagnetic particles comprising .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, both of which may be used in pure form or with the incorporation of Co, CrO.sub.2, etc. Such a magnetic coating layer has a coercive force of about 250 to 600 Oe and residual magnetization of about 1,000-1,700 Gauss.
New magnetic coating layer formulations comprising ferro-magnetic alloys dispersed in a binder have been proposed such as those set forth in Japanese Patent Applications (OPI) Nos. 43111/1976, 65905/1976, 97800/1976, 101502/1976, 140200/1976, 56508/1977, 56511/1977, 108804/1977 (which corresponds to Canadian Pat. No. 959,979 and OLS No. 2,710,268), 11967/1978 and 13906/1978 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".), Japanese Patent Application No. 154491/1976, etc. A magnetic coating layer containing a finely-divided ferromagnetic alloy exhibits a high coercive force as well as a high residual magnetization which is effective to expand the dynamic range (the range between the maximum recorded sound level and the bias noise level) and provide high fidelity recording and reproduction characteristics when used in audio recording. The ferromagnetic alloy coating layers comprising a finely-divided metal alloy dispersed in a binder can be manufactured stably and are capable of providing a magnetic audio-recording material having a coercive force of 800 to 1300 Oe and a residual magnetization of 2500 to 4000 Gauss. In comparison to conventional types of magnetic coating layers, coating layers based on the alloys have more than twice as high coercive force and residual magnetization, which would lead one to expect an improvement in dynamic range by at least as much as 6 dB. In practical embodiments, the thickness of the magnetic coating layer can be reduced which unfavorably works for the improvement of dynamic range at the lower frequency region but which favorably works at the higher frequency region since the tape coating thickness loss (See, for example, "Radio Technics Series", Vol. 16, "Hi-Fi Tape Recorder", pp. 138, published by Radio Technics Co. (Rajio-gijutsu-sha) 1969.) is reduced. Hence the magnetic audio-recording tape comprising a finely-divided ferromagnetic metal alloys dispersed in a binder has markedly improved properties and particularly a wide dynamic range towards high frequency regions.
However, when a music program is recorded in and played back from such an metal alloy tape, the listener does not sense the resulting sound as having an expanded dynamic range primarily because the modulation noise level of the metal alloy based magnetic tape is greater than that of the conventional tapes. The increase in the modulation noise level is presumably attributed to the surface roughness or the non-uniformity of the magnetic coating layer, which is suitable for conventional coating formulations but which, when combined with the higher residual magnetization of metal alloy coatings, gives rise to an unacceptable level of magnetic fluctuation.
Further experiments have revealed that modulation noise can be effectively suppressed by improving the uniformity of either the surface of the magnetic coating which faces the magnetic head or the surface contiguous to the tape support. Specifically it has been confirmed that, in the manufacture of a tape by coating a dispersion of a finely-divided magnetic material in a binder on a support and then drying, the surface uniformity of the magnetic coating can be markedly improved by improving the smoothness of the support surface on which the magnetic coating layer is to be provided.